While keratometry is understood to be the measurement of the curvature of the cornea, topography relates to the three-dimensional measurement of geometric surfaces, in our case of the cornea of an eye.
The measurement of cornea curvature normally occurs in that the cornea is illuminated in a structured manner, and the light beams reflected by the cornea are detected. According to the prior art, two different optical approaches are known for this.
In the first, more traditional approach, the cornea is illuminated with structures, e.g. individual light structures or Placido ring systems, and the resulting images are viewed with conventional optical imaging systems. Because of the type of lighting, in order to obtain a precise evaluation of the reflected images it is absolutely necessary to determine the distance between the eye or cornea and the measurement device. Appropriate assemblies for this shall be explained in brief below.
With the method known for some time, and used substantially in so-called keratometers or keratographs, individual light structures or Placido rings are imaged reflection from the precorneal tear film, and the reflected signals are observed with imaging optics or recorded with a camera and evaluated. Depending on the curvature of the cornea, the pattern that is reflected and detected by the camera is scaled in size. In order to obtain a determination of the curvature from these reflection signals, the size of the reflected pattern must be compared with a known shape, normally resulting in a sphere having a radius of 7.8 mm. A solution of this type is described, for example, in U.S. Pat. No. 4,684,140 A.
The Placido disks used in topographs for generating concentric rings are not necessarily a planar disk. Although such planar Placido disks are sufficiently known in the prior art, and described, for example, in U.S. Pat. Nos. 5,110,200 A and 5,194,882 A, funnel-shaped (U.S. Pat. Nos. 5,684,562 A, 6,116,738 A) or spherically curved (U.S. Pat. No. 5,864,383 A) Placido disks, are more commonly used.
Solutions for ophthalmometers (also keratometers) are described in U.S. Pat. Nos. 6,575,573 B2 and 6,692,126 B1, which are supplemented by slit lamp projectors. While the imaging of Placido ring systems is provided for the measuring of the surface curvature of the cornea of the eye, sectional images of the eye are generated with the slit lamp projectors, from which the thickness of the cornea of the eye can be determined. As a result of this combination, a cornea thickness profile can be determined.
One disadvantage of these types of solutions can be seen in that the precision of the measurement is strongly dependent on the angular relationships, and thus on the measurement distance. Very different methods are used to determine, or check for, the correct measurement distance. As such, the measurement can be automatically triggered when the correct working distance has been reached. On one hand, this can occur through a correction of the erroneous distance prior to each measurement, in that the distance, or the position, is determined and, if necessary, corrected, using photo sensors, contacts or additional measurement systems.
By way of example, U.S. Pat. Nos. 6,048,065 A and 6,070,981 A are specified in this regard. The solutions described therein depict topographs based on a Placido disk. For checking for the correct measurement distance, both solutions make use of a point light source, the light from which illuminates the cornea, is reflected therefrom, and reproduced on a CCD camera as a point image. The position of the point image inside the recording range provides information regarding the distance between the Placido disk and the eye. For an exact positioning, the Placido disk is displaced until the distance is optimized. The measurement first begins at this point.
The distance-independent approach enables the projection of numerous points, circles or other suitable patterns with limited technical expenditure. Although a detailed determination of the topography of the cornea is substantially simplified by this, it is disadvantageous thereby, that a relatively complex measurement system for determining the distance to the eye is necessary.
There are approaches that combine established keratometers with a topography measurement device for measuring the cornea. For this, in addition to the existing ring structure, 6 light points are integrated on the Placido disk, for example. The disadvantage with these types of solutions, however, is that the projection of the 6 light points does not occur in the form of parallel beams, i.e. in a distance-independent manner, and for this reason, an additional distance measurement device is needed.
As such, an ophthalmological device in the form of a 2-zone keratometer is known, with which additional topography measurements can be carried out through an attachment. The attachment has the form thereby of a (funnel-shaped) Placido disk. Because the projection of the measurement points likewise does not occur in a distance-independent manner thereby, the implementation of a distance measurement system is needed.
In the second approach, the structures are projected onto the cornea from the infinite, thus in the form of parallel beams, and the image reflected from the cornea is observed with a telecentric lens assembly. Because of the projection of parallel beams, this approach is distance-independent, such that the determination of the distance between the eye and the measurement device is unnecessary.
In addition, there is an approach described in WO2000/33729 A2, in which 6 punctiform structures are projected onto the cornea from the infinite using 6 separate lenses.
U.S. Pat. No. 4,660,946 A describes a solution for measuring the shape of the cornea based on a disk-shaped Fresnel cylindrical lens. Each ring of the Fresnel cylindrical lens is annularly illuminated individually by application of annular cylindrical lenses. On one hand, the number of implementable rings is limited by the disk-shaped structure, and on the other hand, it becomes increasingly difficult to implement this type of lighting as the number of rings increases.
Another solution is described in WO 2012/160049 A1. Therein, an element in the form of a Fresnel axicon lens is disposed in the beam path, and illuminated by a lighting unit over its entire surface with plane waves. Aside from in the region of the telecentric distance-independent image capturing, the Fresnel axicon has annular structures of different radii. Although this solution enables the generation of numerous parallel beams having different angles of incidence, very high demands, however, in terms of precision, are placed on its production as well as the adjustment thereof. Furthermore, this solution requires a great deal of space, making it difficult to integrate it in multifunctional ophthalmological devices.